WordPress Plugin Activation Error: Why 'Unexpected Output' and How to Fix It?

Unexpected Output Error during Plugin Activation in WordPress

When activating a plugin in WordPress, you may encounter the message: "The plugin generated unexpected output during activation." This issue arises when external output occurs beyond the plugin initialization area.

Causes of the Error

1. Incorrect Output Placement: Attempting to display output (e.g., echo) outside of designated hooks or outside the plugin initialization process.
2. PHP Errors: An uncaught PHP error can trigger this message.

Solutions

1. Verify Output Location:

• Use appropriate hooks for displaying messages in the admin area (e.g., admin_notices).
• Output data only in designated hooks or areas in the plugin's initialization process.

2. Debug PHP Errors:

• Utilize the provided PHP debug code snippet to capture and display PHP errors that may be causing the issue.

  define('temp_file', ABSPATH.'/_temp_out.txt' );
  
  add_action("activated_plugin", "activation_handler1");
  function activation_handler1(){
    $cont = ob_get_contents();
    if(!empty($cont)) file_put_contents(temp_file, $cont );
  }
  
  add_action( "pre_current_active_plugins", "pre_output1" );
  function pre_output1($action){
    if(is_admin() && file_exists(temp_file))
    {
        $cont= file_get_contents(temp_file);
        if(!empty($cont))
        {
            echo '<div class=&quot;error&quot;> Error Message:' . $cont . '</div>';
            @unlink(temp_file);
        }
    }
  }

By addressing the underlying issue, you can effectively activate your plugin without encountering the "unexpected output" error.

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